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The extensional Val Roveto Fault, which is the longest exhumed potentially-seismogenic structure of central Apennines, Italy, is examined to constrain earthquake-related fluid circulation and fluid sources within shallow carbonate-hosted faults. The study focuses on fault-related comb and slip-parallel veins that are calcite-filled and cut through the principal surface of the Val Roveto Fault. We observe multiple crack-and-seal events characterized by several veining episodes, probably related to different slip increments along the fault plane. We show that vein calcite precipitated in Late Pleistocene time below the present-day outcrop level at a maximum depth of ∼350 m and temperatures between 32 and 64◦C from meteoric-derived fluids modified by reactions with crustal rocks and with a mantle contribution (up to ∼39%). The observed warm temperatures are not compatible with a shallow (≤∼350m) precipitation depth, which, in this region, is dominated by circulation of cold meteoric water and/or shallow groundwater. Based on structural–geochemical data, we propose that deep-seated crust–mantle-derived warm fluids were squeezed upward during earthquakes and were hence responsible for calcite precipitation at shallow depths in co-seismic comb and slip-parallel fractures. As comb-and slip-parallel veins are rather common, particularly along seismogenic extensional faults, we suggest that further studies are necessary to test whether these veins are often of co-seismic origin. If so, they may become a unique and irreplaceable tool to unravel the seismic history of hazardous active faults.